Cavity backed spiral antenna

ABSTRACT

Cavity backed spiral antenna apparatus having an extremely narrow depth. The spiral antenna elements are preferably equiangular and comprise less than a full turn but more than a half turn and are ferrite loaded to result in a cavity depth which is on the order of 1/80 of the wavelength of the highest frequency of interest.

JH-Q-GILQ DH OL-ZQ-73 OR 397179877 0 United States Patent 1 1 11113,717,877 Hanninen et a]. 1 Feb. 20, 1973 [s41 CAVITY BACKED SPIRALANTENNA 3,310,307 3/1967 Kofoid ..343/787 [75] Inventors: Albert E.Hanninen, Londonderry; 3322 S5222; Shaun scanlon Nashua 31544920 1/19701401115111 1 3,555,554 1/1971 Kuo ..333/83 [73] Assignee: ISqanPtIlersAssociates, Inc., Nashua, Primary ExaminermE Lieberman Attorney-LouisEtlinger [22] Filed: Feb. 27, 1970 21 Appl. No.: 15,102 [57] ABSTRACTCavity backed spiral antenna apparatus having an ex- [52] U 5 Cl 343/872343/895 tremely narrow depth. The spiral antenna elements [51] q 1/36are preferably equiangular and comprise less than a full mm but morethan a half mm and are ferrite 9 [58] Field of Search 343/787 789 8 5872 loaded to result in a cavity depth which 1s on the order [56]Reterences cued of 1/80 of the wavelength of the highest frequency ofUNITED STATES PATENTS Russell ..343/895 11/1968 Meyers ..343/787'IIIIIIIII YIIIIIII/A interest.

3 Claims, 4 Drawing Figures 1 CAVITY BACKED SPIRAL ANTENNA The inventionherein described was made on the course of or under a contract orsubcontract thereunder with the Department of the Navy.

BACKGROUND OF INVENTION This invention relates to new and improvedantennas and in particular to cavity antennas which are of relativelysmall physical size.

Electrically small cavity antennas are useful in any applicationrequiring compact physical dimensions, as for example in airborne,missile, space and similar environments. One type of antenna which hasbeen used in the past for applications such as the foregoing where sizeand weight are critical, is the so-called spiral type antenna described,for example, in U. S. Pat. No. 2,863,145, issued Dec. 2, 1958 to E. M.Turner. Although such spiral type antennas have been adequate in manycases, the cavity depth dimension of the associated cavities have beentoo large for those applications in which the cavity depth must besignificantly smaller than V4 wavelength at the highest frequency ofinterest.

BRIEF SUMMARY OF INVENTION Accordingly, it is an object of the presentinvention to provide a novel and improved cavity type antenna which isof relatively small physical size.

It is another object of the invention to provide a cavity backed spiraltype antenna in which the cavity depth is extremely small compared tothewavelength at the highest frequency of interest.

Briefly, the invention is embodied in antenna apparatus having a cavitywith an aperture. A spiral antenna is mounted over the cavity apertureand is loaded by means of a sheet of ferrite material which is disposedwithin the cavity. In one embodiment, the spiral antenna elements aremounted upon the ferrite sheet. In another embodiment, the spiralelements are mounted upon a radome which is in turn mounted over thecavity aperture. A feed means is provided to couple energy to and fromthe spiral antenna elements. In a preferred embodiment the spiralantenna elements are'equiangw' lar and each comprises less than a fullturn but greater than a half turn. With ferrite loading in thispreferred embodiment, a cavity depth on the order of H100 of thewavelength of the highest frequency of interest is typical.

BRIEF DESCRIPTION OF DRAWINGS In the drawings, like reference charactersdenote like structural elements; and

FIG. 1 is a sectional view taken along the lines I-l of FIG. 2 of acavity backed spiral antenna embodying the invention;

FIG. 2 is a plan view of the spiral antenna elements mounted upon aferrite sheet in accordance with one embodiment of the invention;

FIG. 3 is a plan view of the spiral antenna elements mounted upon aradome in accordance with another embodiment of the present invention;and

FIG. 4 is a sectional view taken along'the lines 4-4 of FIG. 3 andfurther illustratingsaidothe'r embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2,there is shown for purposes of illustration a cavity backed spiralantenna consisting of conducting antenna elements 10 and 11 having feedterminals 13 and 14, respectively. The antenna elements 10 and 11 arecoiled in the configuration of a concentric Archimedian spiral.Preferably the antenna elements 10 and 11 are also equiangular. That is,the width of each element is largest at the center of the spiral andbecomes progressively narrower toward the outside of the spiral. Inaddition, the antenna elements l0 and 11 preferably consist of less thana full turn and slightly greater than a half turn.

According to a first embodiment of the invention the antenna elements 10and 11 are mounted upon a sheet 15 of ferrite material. The sheet 15 hasa geometrical shape so as to fit the interior of a cavity 16. For theillustrated embodiment the geometrical shape of the sheet 15 and of thecavity 16 takes the form of a cylinder or disc. The sheet 15 and thecavity 18 have apertures 17 and 18, respectively, each adapted toreceive a feed cable 19. The feed cable 19 comprises a center conductor23 and an outside conductor 24 which are adapted to connect to the feedterminals 13 and 14, respectively when assembled. The ferrite sheet 15,the cavity 16 and the cable 19 are shown as separate elements in FIG. 1in order to conveniently illustrate the features of each. When fullyassembled, the ferrite sheet 15 fits within the cavity 16 as indicatedby the arrow and the cable 19 fits within the apertures 17 and 18 of thesheet 15 and cavity 16. The sheet 15 may be held within the cavity 16 byany suitable fastening means such as adhesives, and the like.

The antenna elements 10 and 11 may be mounted upon the surface of theferrite sheet 15 by any suitable technique. For example, the elements 10and 11 may be formed by photo etching technique.

The ferrite sheet 15 has a relatively high dielectric constant comparedto that of the conducting elements 10 and 11 or of the conducting cavity16. Consequently, the ferrite sheet 15 is a substantiallyelectromagnetically transparent material.

Referring now to FIGS. 3 and 4 there is shown another embodiment of theinvention in which the spiral antenna elements 10 and 11 are mountedupon a radome 12 rather than upon the surface of the ferrite sheet 15.Radome 12 may be of any suitably electromagnetically transparentmaterial such as epoxy, glass, and the like. In all other respects theembodiment as shown in FIGS. 3 and 4 is identical to that shown in FIGS.1 and 2 and like structural elements bear like reference characters.

ln bothof the above described embodiments of the invention the ferritesheet 15 serves the same purpose, namely, to provide a loading of theequiangular spiral antenna'elements 10 and 11. The particularconfiguration of the equiangular spiral'antenna elements 10 and 11 insomewhat less than a full turn but greater than a half turn with ferriteloading has resulted in a cavity depth w which is on the order of H ofthe wavelength A at the highest frequency of interest. According to onecircularly polarized antenna design embodying the invention and usefulin the ultrahigh frequency (UHF) communications band where the frequencyof 1,000

megahertz is the highest frequency of interest, (k =1 1.8 inches) thecavity depth w =0.l5 inches. For the same design the cavity diameter dis 2.75 inches. In spite of the ferrite loading, the circularity of thepolarization is maintained. It will be appreciated that the abovedescribed cavity backed spiral antenna embodiments of the presentinvention may be employed either for radiating purposes or for receivingpurposes. Consequently, the feed cable 19 is connected at its other endto a transmitter/receiver device (not shown).

it will thus be seen that the objects as set forth above, among thosemade apparent from the preceding description, are efficiently attainedand, certain changes may be made in the illustrated structures withoutdeparting from apparatus which embodies the invention.

It is intended that all matter contained in the above description orshown in the accompanying drawings is primarily illustrative ofapparatus embodying the invention.

What is claimed is:

1. A cavity having an aperture;

a spiral antenna mounted over said aperture, said spiral antennaincluding a pair of conducting elements which are coiled in spiral andare equiangular, each of said elements being less than one turn andgreater than one half turn,

loading means including a sheet of ferrite material mounted within saidcavity whereby the cavity depth is on the order of M where A is thewavelength of the highest frequency of interest; and

feed means for coupling energy to and from the spiral antenna.

2. The invention as set forth in claim 1 wherein said spiral antennaelements are mounted upon a surface of said ferrite sheet which facesthe cavity aperture.

3. The invention as set forth in claim 1 wherein said spiral antennaelements are mounted upon a radome which in turn is mounted over saidcavity aperture.

1. A cavity having an aperture; a spiral antenna mounted over saidaperture, said spiral antenna including a pair of conducting elementswhich are coiled in spiral and are equiangular, each of said elementsbeing less than one turn and greater than one half turn, loading meansincluding a sheet of ferrite material mounted within said cavity wherebythe cavity depth is on the order of lambda /80 where lambda is thewavelength of the highest frequency of interest; and feed means forcoupling energy to and from the spiral antenna.
 1. A cavity having anaperture; a spiral antenna mounted over said aperture, said spiralantenna including a pair of conducting elements which are coiled inspiral and are equiangular, each of said elements being less than oneturn and greater than one half turn, loading means including a sheet offerrite material mounted within said cavity whereby the cavity depth ison the order of lambda /80 where lambda is the wavelength of the highestfrequency of interest; and feed means for coupling energy to and fromthe spiral antenna.
 2. The invention as set forth in claim 1 whereinsaid spiral antenna elements are mounted upon a surface of said ferritesheet which faces the cavity aperture.